1. Field of the Invention
The present invention relates generally to the art of actuator devices which use a shape memory alloy (SMA) and its ability to do work when transforming from martensite to austenite. More specifically, the present invention relates to extending the useful life of such actuators by controlling the amount of martensite strain imposed on the SMA material to below about 3% by limiting the upper stress on the element. Still more specifically, the preferred embodiment of the present invention relates to providing an actuator device with a ribbon shape actuator prepared from a nickel-titanium SMA alloy which is capable of millions of cycles without failure.
2. Description of the Prior Art
Shape memory alloy (SMA) materials are well known. Shape memory alloys are those alloys which undergo a crystalline phase transition upon heating and cooling, and upon application or removal of stress. Normally, the transition from martensite to austenite and austenite to martensite occurs over a temperature range which varies with the composition of the alloy itself and the type of thermal-mechanical processing. When stress is applied to the SMA member in the austenite phase and cooled through the austenite to martensite transition temperature range, the austenite phase transforms to martensite, and the shape of the SMA member changes due to the applied stress. Upon heating, the SMA member returns to its original shape when the martensite transforms to austenite.
A large amount of prior art discusses the alloys themselves and techniques for improving the performance of alloys in certain applications. Nickel-titanium alloys having approximately a 50:50 ratio of these elements is one well known SMA material. Variations of this base alloy are also known. See, for example, U.S. Pat. No. 5,114,504, issued May 19, 1992 to AbuJudom, et al. for "High Transformation Temperature Shape Memory Alloy," and U.S. Pat. No. 5,109,523, issued Apr. 28, 1992 to Peterseim, et al. for "Shape Memory Alloy." Both patents, in their Background sections, provide additional basic information about the shape memory phenomenon and certain techniques for modifying the properties, usually the temperatures at which the relevant transformations take place.
Some attempts have also been made to modify the physical and mechanical properties of SMA, such as those discussed in Thoma, et al., U.S. Pat. No. 4,881,981, issued Nov. 21, 1989 and entitled "Method for Producing a Shape Memory Alloy Member Having Specific Physical and Mechanical Properties." In this patent disclosure, the internal stress of the SMA is increased by cold working, and the member is then formed into the desired configuration. The member is then heat treated at a selected memory imparting temperature. It is also known that the transformation temperatures may be stabilized by cycling the SMA element between martensite and austenite under an applied stress.
A wide variety of uses exist for SMA, including actuators for robotic devices, clamps and fasteners and for other applications where it is desirable to take advantage of the rather dramatic shape changes which accompany the phase transitions under an applied stress. However, one problem with commercialization of SMA devices has been the relatively short useful life of the actuators, for causes which heretofore have not been fully appreciated. It has been difficult to design SMA actuators which can be uniformly heated and cooled and to provide actuators where thermal conduction is even along the actuator element, as opposed to inconsistent, e.g. in areas where the SMA contacts other actuator components such as pulleys and termination elements. In the past, high stress levels were applied to SMA elements during the austenite to martensite transition and the amount of SMA element strain was controlled with mechanical stops. A consequence of the high applied stress is that sections of the SMA element that cool below the austenite to martensite transformation temperature first will undergo martensite straining as much as 8% in the localized cooled section. Such problems are believed to result in SMA element deterioration and a reduction in useful life. SMA element failure occurs in the highly strained sections that cool first.
As an example of such problems, assume the SMA element is being used in a pulley containing switch, and an alloy is selected which may increase by as much as 5-8% in length when it transforms to the martensite phase under stress, compared to its original dimensions in the austenite phase. Problems can result if the element is not uniformly cooled, because the pulley (for example) will act as a heat sink for that portion of the element in contact therewith. The cooling will be uneven as pulleys and other actuator components will cause parts of the element to cool more rapidly or more slowly than others. The reverse situation will occur when heating of element takes place and the material returns to its original shape. This results in premature failure of the element.
Most prior art actuator elements are made from wire which has a circular cross-section, presumably because of ease of fabrication. It is known, however, that ribbons with a rectangular cross-section may be used for SMA elements. Such ribbons are believed to have improved performance due to their lower outer fiber stress when bent and increased surface area-to-volume ratio. Because ribbons with a rectangular cross-section have a greater surface area-to-volume ratio than circular cross-section members, they cool faster. It is also known that heat insulating materials can be used for the pulley and termination contact points to assist in eliminating hot and cold spots and uneven heat transfer to and from the SMA element.
With the known characteristics of SMA alloys and a considerable amount of knowledge about improvements in their physical and mechanical properties, commercialization of this technology has still proceeded slowly. Reliability seems to be a major factor in the slow growth of this exciting technology, and any improvement thereto would constitute a significant advance in the art.